“To help inform the Department’s decision making and the public’s understanding, and to assist the FAA [Federal Aviation Administration] in ensuring that its safety procedures are implemented effectively, this is to confirm my request that the Office of Inspector General proceed with an audit to compile an objective and detailed factual history of the activities that resulted in the certification of the Boeing 737-MAX 8 aircraft,” Chao said in a letter to Inspector General Calvin L. Scovel III.

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Boeing has been the subject of intense scrutiny since two recent crashes involving Max 8 aircraft.

Officials have said black-box data from the two crashes, one involving Ethiopian Airlines and another Lion Air, share a “clear similarity.”

Last week, President Trump announced the U.S. would join other countries in grounding Max 8 and 9 planes while ongoing investigations into the crashes unfold.

The Wall Street Journal reported Monday that federal prosecutors and Transportation Department officials are seeking documents relating to the development of the 737 Max, with the Justice Department probe involving a prosecutor in its fraud division.